Compression/injection molding of polymer-derived fiber reinforced ceramic matrix composite materials

ABSTRACT

Methods of making fiber reinforced ceramic matrix composite (FRCMC) parts by compression and injection molding. The compression molding method generally includes the initial steps of placing a quantity of bulk molding compound into a female die of a mold, and pressing a male die of the mold onto the female die so as to displace the bulk molding compound throughout a cavity formed between the female and male dies, so as to form the part. The injection molding method general includes an initial step of injecting a quantity of bulk molding compound into a cavity of a mold. In both methods, the bulk molding compound is a mixture which includes pre-ceramic resin, fibers, and, if desired, filler materials. Once the part has been formed by either method, the mold is heated at a temperature and for a time associated with the pre-ceramic resin which polymerizes the resin to form a fiber-reinforced polymer composite structure. The part is then removed from the mold, and heated a second time at a temperature and for a time associated with the polymerized resin which pyrolyzes it to form the finished FRCMC structure. These methods can also be modified to allow for the molding of heterogeneous FRCMC parts wherein different portions of the part contain different types of fiber and, if desired, different filler materials, so as to vary the characteristics exhibited by each portion thereof.

This is a division of application Ser. No. 08/704,348, filed Aug. 28,1996, now U.S. Pat. No. 5,738,818.

BACKGROUND

1. Technical Field:

This invention relates to methods of forming structural and mechanicalparts from fiber-reinforced ceramic matrix composite (FRCMC) materials,and more particularly, to the compression and injection molding of theseparts.

2. Background Art:

Composite material structures are very popular for various uses.Typically, these structures constitute a matrix of "cured" organicresins with some type of fiber dispersed throughout. More recently,fiber reinforced ceramic matrix composite (FRCMC) structures have beenmade available for use where due to high temperatures organic compositesare not suitable. A typical FRCMC structure comprises fibers of varioustypes and lengths disposed throughout a ceramic material formed from apre-ceramic resin. While organic composites will burn readily, FRCMC,being a ceramic, withstands heats that can destroy even metals. Forexample, a FRCMC material can withstand continuous temperatures up toabout 1000° F., cyclical temperatures up to about 2000° F., andshort-term exposure to temperatures up to about 3500° F.

FRCMC structures are made by combining the aforementioned pre-ceramicpolymer resin, such as silicon-carboxyl resin sold by Allied Signalunder the trademark BLACKGLAS or alumina silicate resin (commerciallyavailable through Applied Poleramics under the product description CO2),with fibers. Examples of types of fibers which might be employed includealumina, Altex, Nextel 312, Nextel 440, Nextel 510, Nextel 550, siliconnitride, silicon carbide, HPZ, graphite, carbon, or peat. These fiberscan be supplied in rigid or binderized preforms, woven or braidedpreforms, random mat preforms, fabric, tow (thread), or chopped tow orfabric. The resin-fiber mixture is formed into the shape of the desiredstructure and heated for a time to a temperature, as specified by thematerial suppliers (typically between 1,500° F. and 2,000° F.), whichcauses the resin to convert into a ceramic material.

There are many methods which can be used to form the FRCMC structures.For example, a resin transfer molding (RTM) process is described in aco-pending application entitled METHODS AND APPARATUS FOR MAKING CERAMICMATRIX COMPOSITE LINED AUTOMOBILE PARTS AND FIBER REINFORCED CERAMICMATRIX COMPOSITE AUTOMOBILE PARTS by the inventors herein and assignedto the common assignee of the present application. This co-pendingapplication was filed on Aug. 16, 1995 and assigned serial number08/515,849. The RTM method described in the co-pending applicationgenerally involves forming a preform in the shape of the part from theaforementioned fibers; placing the preform in a cavity of a mold havingthe shape of the part; forcing a liquid pre-ceramic polymer resinthrough the cavity to fill the cavity and saturate the preform; heatingthe mold at a temperature and for a time associated with the pre-ceramicpolymer resin which transforms the liquid pre-ceramic polymerresin-saturated preform into a polymer composite part; removing thepolymer composite part from the mold; and, firing the polymer compositepart in a controlled atmosphere at a temperature and for a timeassociated with the pre-ceramic polymer which transforms it into aceramic, whereby the polymer composite part is transformed into a fiberreinforced ceramic matrix composite part.

The RTM method of forming FRCMC structures works well for its intendedpurpose. However, this method requires the use of a fiber preform whichmust be placed in the mold prior to the structure being formed. Thesepreforms add to the expense of producing the FRCMC structure, not onlydue to the cost of the preform itself, but also because of the extraprocessing steps required to install the preform into the mold.

Accordingly, there is a need for a method of making FRCMC parts that isconducive to mass producing these parts at a faster pace and at areduced cost in comparison to other methods, such as RTM processes.

Wherefore, it is an object of the present invention to provide such amethod of making FRCMC parts via a compression molding process whereininstead of having to employ fiber preforms such as with a RTM process,chopped fibers are mixed with a pre-ceramic resin prior to placing themixture into a compression mold.

Wherefore, it is another object of the present invention to provide sucha method of making FRCMC parts via an injection molding process whereininstead of having to employ fiber preforms such as with a RTM process,chopped fibers are mixed with a pre-ceramic resin prior to beinginjected into an injection mold.

It is also an object of the present invention to provide such a methodof making FRCMC parts having heterogeneous structures wherein variousportions of the part included different fiber types and potentiallydifferent filler materials which impart characteristics desired to beexhibited by a particular portion of the part.

SUMMARY

The above-described objectives are realized with embodiments of thepresent invention directed to methods of making fiber reinforced ceramicmatrix composite (FRCMC) parts by compression and injection molding. Thecompression molding methods generally include the steps of:

(a) Placing a quantity of bulk molding compound into a female die of amold. The bulk molding compound is a mixture which includes pre-ceramicresin and fibers.

(b) Pressing a male die of the mold onto the female die so as todisplace the bulk molding compound throughout a cavity formed betweenthe female and male dies. The walls of the cavity form the exteriorsurfaces of the FRCMC part being molded.

(c) Heating the mold at a temperature and for a time associated with thepre-ceramic resin which polymerizes the resin to form a fiber-reinforcedpolymer composite structure.

(d) Removing the polymerized composite structure from the mold.

(e) And, heating the polymerized composite structure at a temperatureand for a time associated with the polymerized resin which pyrolyzes itto form a FRCMC structure.

Whereas, the injection molding methods generally include the steps of:

(a) Injecting a quantity of bulk molding compound into a cavity of amold. Here too, the bulk molding compound is a mixture which includespre-ceramic resin and fibers, and the cavity walls form the exteriorsurfaces of the FRCMC part being molded.

(c) Heating the mold at a temperature and for a time associated with thepre-ceramic resin which polymerizes the resin to form a fiber-reinforcedpolymer composite structure.

(d) Removing the polymerized composite structure from the mold.

(e) And, heating the polymerized composite structure at a temperatureand for a time associated with the polymerized resin which pyrolyzes itto form a FRCMC structure.

In either of these molding methods, it is preferred that the bulkmolding compound specifically include a quantity of fibers whichcorresponds to the maximum percent by volume of fibers capable of beingdispersed throughout the FRCMC part. This maximum is between about 15and 50 percent (and possibly more) dependent on the length of the fibersemployed, the shape of the part being molded, and the quantity of fillermaterial (if any) that is added to the compound. In addition, the moldpreferably has one or more resin outlet ports connecting the cavity ofthe mold to its exterior. These outlet ports are used to allow anyexcess resin exceeding that required to form the part to be expelled.Each resin outlet port has a cross-sectional area small enough tosubstantially ensure the fibers cannot flow through the port, yet largeenough to allow the flow of resin therethrough. The bulk moldingcompound will preferably have such excess amounts of resin to ensure thefibers flow with the resin within the cavity of the mold at itsprescribed viscosity. The prescribed viscosity of the pre-ceramic resinis chosen such that it is high enough to ensure the aforementioned fiberflow, but low enough to ensure the resin readily flows through thefibers once packed into position, with any excess flowing out of theoutlet ports. It is noted that a greater viscosity is required forlonger fibers. For a selected part, preferably the fibers areapproximately the same length. Fiber lengths can range from about 0.125inch to 12 inches in length. For fibers within the preferred range, theprescribed viscosity of the pre-ceramic resin will preferably rangebetween 5000 and 30,000 centipoise, depending on the length of thefibers. As alluded to above, the bulk molding compound may also includea quantity of filler material which corresponds to the percent by volumeof filler material desired to be dispersed throughout the finished FRCMCpart.

Once the FRCMC part is molded via either the compression or injectionmolding processes of the present invention, it is preferred that anadditional procedure be performed to eliminate pores created during therequired heating cycles. Eliminating these pores strengthens the part.Specifically, after the completion of the heating step which pyrolyzesthe FRCMC part, the part is immersed into a bath of a pre-ceramic resinto fill the pores. The part is then heated at a temperature and for atime associated with the resin filling said pores so as to transform itto a ceramic material. Unfortunately, the process of heating the resinfilling the pores will create further pores. Accordingly, it is desiredthat the filling and heating steps be repeated until the pore densitywithin the FRCMC part is less than a prescribed percentage by volume.This prescribed percentage corresponds to the point where the part willexhibit a repeatable strength from one part to the next. It is believedfive iterations of the filling and firing process are required to obtainthis repeatable part strength. To facilitate the filling step, it ispreferred that the resin has a water-like viscosity. In addition, theFRCMC part could be placed in a vacuum environment to assist in thefilling of the pores.

The compression and injection molding processes of the present inventionare capable of producing a FRCMC part having a homogeneous structurewhere the percentages of resin, fiber and filler (if present) will besubstantially the same throughout the molded part. However, theseprocesses can be modified to produce parts having a heterogeneousstructure where the types and percentages of the components making upthe composite vary from section to section in order to impart a varyingset of characteristics (i.e. physical, electrical, etc.). This can beaccomplished in an embodiment of the compression molding process of thepresent invention by employing a layering process. The layering involvesplacing a sheet of fiber cloth on top of the quantity of bulk moldingcompound placed into a female die of a mold prior to closing the mold.Fiber cloth is a material typically made up of long intermingled orwoven fibers. When the mold is closed, the resin from the bulk moldingcompound will flow into the sheet of fiber cloth. Thus, once the resinis pyrolyzed, the resulting part will have one layer exhibitingcharacteristics imparted by the fiber cloth and one layer exhibitingcharacteristics imparted by the fibers (and possibly fillers) present inthe bulk molding compound, all integrated by the ceramic matrix materialpresent throughout the structure. If the fiber cloth is dense, as isusually the case, it will form a barrier to the fibers and fillermaterials (if any) present in the bulk molding compound, thereby keepingmost of them in the bulk molding compound layer. However, this samedensity can prevent a complete saturation of the fiber cloth sheet bythe resin and cause unwanted voids in the completed FRCMC part.Accordingly, it is preferred that the fiber cloth sheet be saturatedwith preceramic resin prior to being placed in the mold. An additionallayer can also be formed by adding another quantity of bulk moldingcompound on top of the fiber cloth before closing the mold. If desiredeven more layers can be incorporated by placing additional alternatinglayers of fiber cloth and bulk molding compound in the mold.

The injection molding process of the present invention can also bemodified to create a heterogeneous FRCMC structure. This is accomplishedby the use of multiple charges of bulk molding compound where eachcharge contains the fibers and filler materials (if used) which willimpart the desired characteristics to a portion of the finished part.Specifically, a separate charge of bulk molding compound is used to formeach portion of the part that is to have differing characteristics froman adjacent portion. This is done by sequentially injecting separatecharges of bulk molding compound into a cavity of a mold until saidcavity is completely packed with fibers. Each charge includes thequantity of fibers which corresponds to the maximum percent by volume offibers capable of being dispersed within the portion of the FRCMC partassociated with the particular charge. Each charge may also include thequantity of filler materials desired to be present in the portion of thepart associated with the charge. Further, it is important that excessresin, over that required to fill the space in the portion associatedwith the charge which is not filled with fibers or filler materials, beincluded in each charge. Specifically, there should be at least enoughexcess resin to fill the portion of the cavity behind the target portionassociated with the charge.

The above-described sequential injection of the various charges of bulkmolding compound is done as follows. The first charge is injected intothe mold. This will result in some portion of the fibers and fillermaterials (if present) associated with that charge being packed into thetarget portion of the part, which in this case will be in the region ofthe mold cavity furthest from the sprue channel(s) of the mold. Oncethis first charge is fully injected, the region of the mold cavitybehind the first target portion will be filled with the remaining resinof the charge, as well as any yet to be packed fibers and fillermaterials. The second charge is then injected into the mold. This chargewill first push the remaining resin, fibers, and filler materials of thefirst charge into the first target portion, thereby packing theremaining fibers and filler materials and forcing the excess resin outof the resin outlet ports. The fibers and filler materials (if any)present in the second charge will then pack into place in the secondtarget portion behind the packed fibers and filler materials in thefirst portion. This process is repeated, if there are to be more thantwo portions, until all the charges have been injected. It is noted thatthe last charge will result in the remaining region of the mold cavityadjacent the sprue channels being packed with fibers and fillermaterials, thus completing the molding of the FRCMC part.

The injection molding process of the present invention which forms aheterogeneous FRCMC structure can be further facilitated by employing a"timed exit" approach. This approach entails incorporating one or moreadditional sets of resin outlet ports. Each one of these additionalresin outlet port sets is disposed at a different location in the cavityof the mold between the first set (which is at the end of the cavityfurthest from the sprue channel(s)) and sprue channel(s). Specifically,these locations corresponding to the end of each of the aforementionedportions of the FRCMC part which is furthest from the sprue channel(s).During the sequential injection process, only the first resin outletport set is open, while the ports of all the other sets are closed. Atthe point where all the fibers and filler materials (if any) associatedwith the first charge are packed into the first portion of the part, theset of ports adjacent the end of the second portion are opened. Thisfacilitates the flow of excess resin out of the mold and speeds up thepacking process in the second portion. It also minimizes anyinfiltration of fibers and filler materials from the second charge intothe first portion of the part. As additional charges are injected intothe mold, the resin outlet ports associated with the portion of the partbeing formed with that charge, are opened. In addition, it is preferablethat the ports associated with a completed portion of the part be closedwhen the ports associated with an adjacent portion are opened, tofurther minimize the aforementioned infiltration.

The "timed exit" approach can also be used to facilitate the injectionmolding of extremely long homogeneous FRCMC parts. An additional set, orsets, of resin outlet ports are incorporated into the mold between thefirst set (most remote from the sprue channel(s)) and the spruechannel(s). The process is essentially the same as described forproducing heterogeneous parts, except that there is only one chargeinjected, and the various sets of ports are opened whenever fibers andfiller materials (if any) pack to a point just ahead of a closed set.

In addition to the just described benefits, other objectives andadvantages of the present invention will become apparent from thedetailed description which follows hereinafter when taken in conjunctionwith the drawing figures which accompany it.

DESCRIPTION OF THE DRAWINGS

The specific features, aspects, and advantages of the present inventionwill become better understood with regard to the following description,appended claims, and accompanying drawings where:

FIG. 1 is a partially cross-sectional view of an apparatus capable ofperforming compression molding of fiber-reinforced ceramic matrixcomposite (FRCMC) parts in accordance with the methods of the presentinvention.

FIG. 2 is a block diagram of a method for the compression molding ofFRCMC parts in accordance with the methods of the present inventionusing the apparatus of FIG. 1.

FIG. 3 is a partially cross-sectional view of an apparatus capable ofperforming injection molding of FRCMC parts in accordance with themethods of the present invention.

FIG. 4 is a block diagram of a method for the injection molding of FRCMCparts in accordance with the methods of the present invention using theapparatus of FIG. 3.

FIG. 5 is a block diagram of a method for the compression molding ofFRCMC parts in accordance with the present invention wherein the moldedpart has a heterogeneous structure.

FIG. 6 is a block diagram of a method for the injection molding of FRCMCparts in accordance with the present invention wherein the molded parthas a heterogeneous structure.

FIG. 7 is a cross-sectional view of an injection mold having multiplesets of resin outlet ports

FIG. 8 is a block diagram of a "timed exit" method of injection moldingin accordance with the present invention using a mold such as that ofFIG. 7.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following description of the preferred embodiments of the presentinvention, reference is made to the accompanying drawings which form apart hereof, and in which is shown by way of illustration specificembodiments in which the invention may be practiced. It is understoodthat other embodiments may be utilized and structural changes may bemade without departing from the scope of the present invention.

The present invention involves two related methods of producing avariety of structural and mechanical parts molded from polymer-derivedfiber reinforced ceramic matrix composite (FRCMC) materials.Specifically, these parts are made by compression or injection molding.

FIG. 1 is a simplified depiction of an embodiment exemplifying theportion of the present invention corresponding to the compressionmolding of polymer-derived FRCMC parts. Generally, a quantity of a bulkmolding compound called a "charge" is placed into a compression mold 10.This compound is a mixture of unpolymerized pre-ceramic resin, choppedfibers, and possible other powderized filler materials. The compressionmold 10 has a female die 12 and male die 14 which when placed togetherform a cavity 16 in-between. The walls of the cavity 16 form thesurfaces of the part being molded when it is filled with theaforementioned compound. In one preferred version of the compressionmolding process, the female die 12 is attached to the bed 18 of aconventional press, while the male die 14 is attached to the ram 20 ofthe press. The two dies 12, 14 are held in alignment with each other bythe bed 18 and the ram 20 such that when the press is activated the diescome together and form the previously-described internal cavity 16. Asdepicted in FIG. 2, the first step 102 in producing a compression moldedpart entails placing a charge of the bulk molding compound in the femaledie while the two dies are separated from each other. The male die isthen pressed down onto the female die to form the part being molded inthe cavity (step 104). Once the molded part is formed, the FRCMCmaterial is heated in the mold (step 106) to a level and for a timesufficient to polymerize the resin, as suggested by the manufacturer ofthe resin. The result is a part similar to bisque-ware in ceramics suchthat it does not have its full strength as yet, but can be handled. Thedies are then separated and the polymerized part is removed from themold (step 108).

FIG. 3 is a simplified depiction of an embodiment exemplifying theportion of the present invention corresponding to the injection moldingof polymer-derived FRCMC parts. In this embodiment the charge of theaforementioned mixture of pre-ceramic resin, chopped fibers, and fillermaterial (if used) is injected into a mold 22, which in the illustratedcase forms a piston within its cavity 24. The mold 22 shown is asimplified two piece unit having a male die 26 which forms the interiorsurface of the piston, and a female die 28 which forms the exteriorsurface of the piston. The female die 28 is attached to a fixed platen30, while the male die 26 is attached to a movable platen 32. Themovable platen 32 can be translated along a set of tie bars 34 by apiston 36. The piston 36 retracts to open the mold 22 to facilitateremoval of a formed part, and extends to clamp the mold 22 closed sothat the bulk molding compound can be injected into the internal cavity24 of the mold. The male and female dies 26, 28 are held in alignmentwith each other by the platens 30, 32 such that when they are clampedtogether by extension of the piston 36, the dies form thepreviously-described internal cavity 24. The female die 28 includes asprue channel 38 which connects the back side of the die to the cavity24. The bulk molding compound is injected into the cavity 24 throughthis sprue channel 38 from a plunger-operated injection assembly 40. Theinjection assembly 40 has an injection tip 42 which interfaces with apassageway through the fixed platen 30. An opening in the end of theinjection tip 42 aligns with the sprue channel 38. The injectionassembly 40 also has a hollow barrel 44 which is connected to theinjection tip 42. An extendible and retractable plunger 46 is disposedwithin the barrel 44 to inject the bulk molding compound into the mold22. The compound is loaded into the barrel 44 through an inlet 48. Asillustrated in FIG. 4, the first step 202 in producing an injectedmolded part entails placing a charge of the bulk molding compound in thebarrel of the injection assembly via the inlet. This is done when theplunger is fully retracted. The male die is then clamped against thefemale die by extending the piston attached to the male die (step 204).Enough force is exerted on the dies by the piston so as to prevent thedies from separating under the pressure of the bulk molding compoundwhen it is injected into the mold cavity. The barrel inlet is thensealed, and the plunger extended to force the bulk molding compoundthrough the injection tip and the adjacent sprue channel, and into thecavity of the mold (step 206). This molds the part which in theillustrated case is a piston . Once the molded part is formed, the FRCMCmaterial is heated in the mold (step 208) to a level and for a timesufficient to polymerize the resin, as suggested by the manufacturer ofthe resin. The piston is then retracted to separate the dies and thepolymerized part is then removed from the mold (step 210).

In either the above-described compression molding or the injectionmolding process, the polymerized part must be heated a second time so asto pyrolyze the polymerized resin (step 110 in FIG. 2 and step 212 inFIG. 4). The manufacturer's recommended procedures should be followed inregards to the heating cycles and any special environmental conditionsthat must be observed.

The pyrolization process which turns the polymer to ceramic causes theformation of pores during the heating cycles. The resultant ceramic partis about 70% solid and 30% pores. To substantially eliminate these poresand thereby strengthen the part, the formed part is immersed in liquidpre-ceramic resin. A vacuum infiltration process may also be used forthis step. In addition, the resin preferably has a very low, almostwater-like, viscosity (e.g. 1-10 centipoise) so that it readily fillsthe pores in the part. The part is then heated once again for the timeand at the temperature indicated by the manufacturer of the resin viathe above described process in order to pyrolyze the resin, thus fillingthe pores.

However, once again the pyrolization process will cause to formation ofmore pores in the previously filled pores (i.e. approximately 30%).Accordingly, the pore filling process is preferably repeated until poreremoval has achieved a desired level. It has been found that after aboutfive iterations of the pore filling process, the resultant part is about95-98 percent solid and will exhibit repeatable high strengths.Accordingly, it is preferred that the process be repeated at least fivetimes, or until a predetermined porosity is achieved.

A key advantage of parts made from FRCMC materials, over conventionalmonolithic ceramic parts, is the added strength and ductility affordedby the fibers. Generally, up to practical limits, the higher thepercentage of fiber by volume in a FRCMC material, the greater itsstrength and ductility. Accordingly, it is desirable to maximize thevolume percent of fiber. However, this presents a problem when formingFRCMC parts via compression or injection molding methods. Specifically,the fibers must be able to flow with the uncured pre-ceramic resin intoevery recess of the mold. Resin rich areas will not exhibit the desiredstrength and ductility. However, as the percentage of fiber increases incomparison to the percentage of resin in the bulk molding compound, theharder it is for the fibers to flow along with the resin. If the resinand fibers where mixed together in the exact quantities and proportionsrequired to form the finished part with the desired high strength andductility, it is possible that the fibers would bunch up in some partsof the mold and thus not flow with the resin to other parts. This wouldcreate undesirable resin-rich areas. One way to ameliorate the fiberflow problem is to increase the viscosity of the resin. A thicker resinwill drag more fibers along than a thinner resin. However, increasingthe viscosity of the resin will work only up to a point. For example, inthe case of injection molding of fiber reinforced plastic composites,the resin is typically made thicker to move the fiber during theinjection process. However, this method can achieve fiber-to-resinratios of only about 5 or 6 percent, even when the resin is madeextremely thick, i.e. on the order of 100,000 centipoise. Fiberpercentages of 5 or 6 percent in a FRCMC material would not produce amolded part with the strength and ductility required for manyapplications. Rather, fiber percentages between about 15 to 50 percentby volume, or more, are preferred for most FRCMC applications. Not onlyis the fiber percentage produced by the "thick resin" method too low forFRCMC material applications, but injecting or compressing such a thickresin presents other problems as well. For instance, special heavy-dutymolding equipment capable of producing the extremely high pressuresrequired to move the resin must be employed. In addition, the molds haveto be strong enough to withstand this high pressure.

The present invention avoids the above-described problems and producesFRCMC parts with the desired higher fiber percentages by taking acompletely different approach. The present invention foregoes theconventional method of trying to load the desired fiber volume into theexact amount of resin required to form the part, and increasing thethickness of the resin in order to move the fiber along with the resin.Instead, the charge (referred to in step 102 of FIG. 2 and step 202 inFIG. 4) is prepared by combining the amount of fiber which whencompressed or injected into the mold would provide the desiredpercentage by volume in the finished FRCMC part, with amounts of resinexceeding that required to form the part. As stated above, the higherthe ratio of fibers to resin, the harder it is to get the fibers to flowwith the resin. By increasing the amount of resin, the fiber-to-resinratio is lowered, thereby making it easier for the fibers to flow withthe resin. Accordingly, the resin does not have to be made excessivelythick to flow the fibers into all parts of the mold. In order to removethe excess resin from the mold, a series of resin outlet ports 50 areincorporated. Examples of these ports 50 are shown in dashed lines inFIGS. 1 and 3. The ports 50 are preferably small enough such that thelength of fiber used in the resin-fiber mixture does not easily passthrough the ports, but large enough that the resin flows throughreadily. In addition, it is preferable that the resin outlet ports 50are placed at points in the mold which are the furthest away from thepoint of contact of the charge and the male die in the case ofcompression molding, and the sprue channel in the case of injectionmolding, so that the fiber will tend to pack from theses remotelocations back toward the male die or sprue channel as the case may be.As the resin-fiber mixture flows through the mold, the excess resin isejected through the resin outlet ports 50, while at the same time thefibers pack within the mold. Once the mold is completely compressed, orall the resin-fiber mixture has been injected into the injection mold,what remains in the mold is the desired percentage of uniformly packedfibers and the remaining resin which has not been ejected. Theabovedescribed molding processes need not be wasteful in regard to theresin that is ejected from the mold. This material can be captured andreused in the next charge to form another part.

In essence, the above-described processes inherently produce the maximumfiber percentage possible. However, there are factors which will limitthis maximum fiber percentage. The length of the fibers and the shape ofthe FRCMC part being formed will in part dictate the highest percent byvolume of fiber-which can be uniformly packed into the mold. Forexample, a complex part having sections with relatively thin walls, orcurved sections having relatively extreme radii, will limit the maximumfiber percentage. This limitation stems from the tendency of the fibersto become trapped in thin wall and curved areas. As a result, the fibersbegin to collect at these areas and limit the amount of fiber that flowswith the resin past the thin wall or curved regions. Longer fibersfurther exacerbate this problem because they tend to become more easilytrapped in the thin wall and curved areas. Therefore, FRCMC parts withthese complex shapes will tend to exhibit lower maximum fiberpercentages than parts having simpler structures. For example, complexparts having thin wall or sharply curved sections will exhibit fiberpercentages closer to the 15 percent by volume. Whereas, FRCMC partshaving simpler structures will exhibit fiber percentages closer the 50or more percent by volume.

It is noted that the ports 50 depicted in FIGS. 1 and 3 aresimplification of the actual resin outlet ports that would be employedin a mold. The exact number and location of the ports is a matter ofmold design. However, the factors effecting mold design for thecompression and injection molding processes according to the presentinvention are essentially the same as those effecting well known resintransfer molding (RTM) techniques employed in the molding of organiccomposite material. The molds employed in RTM also often require the useof resin outlet ports. Accordingly, one skilled in the art could readilydesign molds for use with the present invention by employing RTMmethods. As these design considerations are known in the art, a detaileddescription of the mold design process will not be provided herein.However, a mold designer employing these well known mold design methodsshould also take into consideration the aforementioned preference thatthe resin outlet ports used with the present invention have across-sectional area which is small enough to prevent fibers of thelength chosen for the bulk molding compound from exiting the mold cavitywith the resin.

The "excess resin" methods according to the present invention have isbeen found to produce FRCMC parts with up to about 50 percent fibervolumes. This level of fiber volume was accomplished in molding of apiston having relatively thick walls and gently curved sections. Abouttwice as much pre-ceramic resin as is needed to form the part was usedto flow fibers having a length of approximately 2.4 inches. The resinhad a resin viscosity of about 8,000 to 12,000 centipoise. This resinviscosity is thick enough to drag the fibers along with the resin, butthin enough to squeeze past fibers that are packed into place within themold on its way towards the resin outlet ports 50. It is anticipatedthat even higher fiber volumes may be possible using the methods of thepresent invention.

The viscosity of the pre-ceramic resin associated with the molding ofthe piston was for the most part dictated by the length of the fiber. Itis believed that shorter fibers can be successfully moved through themold with a less viscous resin. For example, it is believed a viscosityas low as about 5,000 centipoise could be employed with fibers havinglength of about 0.125 inches. Conversely, a thicker resin will berequired to move longer fibers through the mold. For example, it isbelieved fibers up to about 12 inches in length can be successfullymoved through a mold using a pre-ceramic resin having a viscosity of nomore than about 30,000 centipoise.

Some FRCMC material applications require the addition of fillers intothe resin-fiber matrix. These fillers are advantageously employed totailor the mechanical, electrical, and other characteristics exhibitedby a molded part. The addition of filler material also has been found tohave process advantages in the context of compression and injectionmolding of FRCMC materials. Namely, the addition of filler materialstends to assist in the moving of the fibers through the mold. As such ithas been found that the viscosity of the resin can be lowered evenfurther, making the molding process quicker as the resin flows throughthe packed fibers more easily. For example, it has been found that theaddition of about 10 percent by volume of 300 mesh silicon carbideparticles into the bulk molding compound used to mold the aforementionedpiston allowed the viscosity of the resin to be lowered to between about3000-5000 centipoise. Filler particles are typically small (i.e. about1-50 microns in diameter) and so flow readily through the mold with theresin and fibers. However, as the fibers begin to pack at the resinoutlet ports, the filler particles tend to lodge within the tangle ofpacked fibers. The resin will continue to flow through the packed fibersand out the resin outlet ports, but most of the filler particles will beleft behind. This process continues as the fibers pack back from theresin outlet ports resulting in a substantially uniform distribution offiller particles throughout the molded part. Thus, the finished FRCMCpart will exhibit a uniform distribution of the filler particles in avolume percentage approximately matching that amount added to theoriginal resin-fiber-filler mixture. It must be noted, however, that theaddition of relatively large quantities of filler materials will lowerthe maximum fiber percentage possible in the molded part. Essentially,the filler materials will physically take up some of the space thatcould have been filled with fiber.

Given the above-described processes, it is preferred that each charge(i.e. the amount of resin-fiber-filler material required to form eachFRCMC part including the excess resin) is preferably made to contain:

a) the amount of fiber which once distributed and packed in the moldwill produce the desired percent volume of fiber. However, the desiredpercentage is limited by the length of the fiber and the complexity ofthe shape of the part being molded, as explained previously. Inaddition, the desired percentage of fiber may be limited by the amountof filler material added to the bulk molding compound;

b) the amount of filler material (if added) which once distributed andpacked in the mold will produce the desired percent volume of filler;and

c) the amount of resin which at a reasonable viscosity will facilitatethe flow of fiber and filler material, but still readily pass aroundpacked fibers and filler material.

The compression and injection molding processes of the present inventiondescribed so far, will produce a FRCMC part which has a homogeneousstructure. In other words the percentages of resin, fiber and filler (ifpresent) will be substantially the same throughout the molded part.However, this need not be the case. Some applications may call for aheterogeneous structure where the types and percentages of thecomponents making up the matrix vary form section to section in order toimpart a varying set of characteristics. This could be accomplished in acompression molding process by employing a layering process. An exampleof this layering process is illustrated in FIG. 5, where a quantity ofpre-mixed bulk molding compound is placed in the bottom of a female molddie (step 302). On top of this layer of molding compound is placed asheet of fiber cloth, i.e. a sheet of material constructed of long wovenfibers (step 304). Next, in step 306, another quantity of bulk moldingcompound is placed on top of the fiber cloth. This process is repeateduntil all the desired layers are formed (step 308). Each layer of bulkmolding compound should include approximately the percentage of fiberand filler volume desired for that layer. The male die is lowered andthe mold compressed to form the part, as discussed earlier. The layersof bulk molding compound are compressed, and the resin flows into thesheets of fiber cloth, and the excess resin is ejected from the moldthrough the resin outlet ports. If the fiber cloth is relatively dense(as it typically would be to maximize ductility and strength of thecloth layer), then the fibers and filler materials present in the bulkmolding compound in a adjacent layer will not readily flow into thecloth. Thus, not only will the difference in fibers vary thecharacteristics exhibited by each layer, but the filler materialspresent in the layers formed from the bulk molding compound will alsoadd to this variation in characteristics. Theoretically, each layer inthe finished part could vary in its characteristics owing to differingfiber types and lengths, and differing fillers and filler quantities.However, the structure itself will be integrated because the ceramicmatrix produced from the pre-ceramic resin will be constant throughoutand so tie each layer together. It is noted that although the resin willflow into a dense fiber cloth, the path of least resistance to the resinflow may be through the outlet ports. Accordingly, it is preferred thatthe fiber cloth be pre-saturated with pre-ceramic resin prior to beingplaced in the mold to ensure there are no voids in the finished partwhich could weaken its structure.

The end result of the layering process would be an integratedmulti-layer FRCMC structure where each layer could potentially exhibitdifferent characteristics (i.e. mechanical, electrical, etc.). Such aFRCMC part can exhibit an overall characteristic not possible using ahomogeneous structure. For example, relatively short, hard fibers couldbe employed in the bulk molding compound placed on either side of athick sheet of fiber cloth made of softer, longer fibers. This wouldresult in a part that has hard, rigid external faces, while stillexhibiting a certain degree of ductility and strength owing to the fibercloth comprising the center of the part.

The creation of a heterogeneous structure in an FRCMC part is alsopossible when using an injection molding process. This can havesignificant advantages in the production of some parts. For example, thepiston referred to previously, would advantageously have an extremelyhard top surface so as to withstand the forced imposed on it within aninternal combustion engine. However, the sides of the piston wouldbenefit from a low coefficient of friction which would facilitate itssliding within a cylinder of an engine. This could be accomplished bymolding the piston so that the fibers and/or filler materials present inthe top of the part impart the desired hardness, while employingdifferent fibers and/or filler materials which impart a degree ofslipperiness in the portion of the piston forming the side walls.Heterogeneous injection molded part structures such as the one justdescribed can be achieved by using multiple charges or partial chargesof bulk molding compound. Specifically, a portion of the part beingmolded which is closer to the resin outlet ports (hereinafter referredto as the first portion) can be made to exhibit differentcharacteristics than an adjacent portion further away from the ports(hereinafter referred to as the second portion). This is accomplished,as illustrated in FIG. 6, by first injecting bulk molding compoundhaving the types of fibers and filler materials which will produced thedesired characteristics in the first portion of the part being molded(step 402). The charge employed preferably contains the percentage offiber and filler material which is desired to be present in the firstportion plus at least enough resin to fill not only the remaining spacein the first portion but also any remaining portion of the mold cavityand associated sprue channel (or sprue channels) behind the firstportion. However, as will usually be the case, a considerable excessamount of resin will also be required to facilitate the movement of thefibers as discussed previously. This excess resin will flow out theresin outlet ports as in the other embodiments of the invention. Oncethe first charge has been completely injected into the mold, asubstantial part of the fibers and filler materials present in thecharge will probably be packed into the first portion. Next, in step404, a second charge containing the quantities and types of fibers andfiller materials desired to be present in the second portion of themolded part is injected into the mold. Here again, the second chargewould also contain enough resin to fill the remaining space in thesecond portion once the fibers and filler material is packed therein, aswell as enough to fill any remaining portion of the mold cavity andassociated sprue channel(s) and any excess required to facilitate theflow of the fibers. The influx of the new bulk molding compound willpush the remaining part of the first charge into the aforementionedfirst portion of the part, thereby packing the remaining fibers andfiller materials (if any) and forcing the remaining excess resin of thefirst charge out of the resin outlet ports. Thereafter, the fibers andfiller materials in the second charge will begin to pack into section ofthe mold corresponding to the aforementioned second portion of the partbehind the packed fibers and filler materials associated with the firstportion. If there are only two portions of the FRCMC part which are toexhibit different characteristics, the second charge will contain enoughfibers and filler material (if any) necessary to finish packing the moldcavity. Once the molding is complete and the part pyrolyzed, and theportions formed by the present method will be integrated by the ceramicwhich will be present throughout the part. This heterogeneous structureinjection molding process may be expanded to create any reasonablenumber of sections having different characteristics. This would beaccomplished by repeating the above-described step associated with thesecond charge for additional charges (optional step 406).

Due to the density of the packed fibers and filler materials in thefirst portion, relatively little of the fibers and filler materials fromthe second charge will infiltrated into the first portion. It isbelieved the only significant infiltration between the first and secondportion will occur in a narrow boundary region between the two. However,this infiltration between sections can be minimized by employing a"timed exit" approach. Essentially, this approach entails opening andclosing appropriately placed resin outlet ports to facilitate theinjection molding of a heterogeneous structure in a FRCMC part. Forexample, as depicted in the mold for a piston shown in FIG. 7, a firstset of resin outlet ports 50 would be disposed adjacent to the far endof the mold cavity 24 (i.e. furthest from the sprue channel 38). Theseports 50 would be open during the part of the above-described injectionprocess corresponding to the packing of fibers and filler materials intothe first portion, as illustrated in step 502 in FIG. 8. This wouldinclude that part of the process where the second charge is used to pushthe remainder of the first charge into the first portion of the moldedpart. Once the second charge reaches the boundary between the first andsecond portions 54, as could be determined by monitoring the volume ofresin ejected from the first outlet ports 50, the first ports 50 wouldbe closed and a second set 56 of previously closed resin outlet portsopened (step 504). This second set of ports 56 is preferably disposed atthe far end of the second portion adjacent the boundary 54 between firstand second portions. The "timed exit" approach avoids having to forcethe resin associated with the second charge through the packed fibersand filler materials in the first portion of the part. This not onlyminimizes the infiltration of fibers and filler materials from thesecond charge into the first portion, but it also speeds up the moldingprocess, as the resin from the second charge will flow more easily outof the second set of outlet ports. It is noted that an open set of portsneed not be closed when opening the next set of ports. However, it ispreferred that a preceding set of ports be closed so as to furtherminimize any infiltration that may take place if an alternate flow paththrough the preceding portion is left in place. It is also noted thatadditional sets of resin outlet ports (not shown in FIG. 7) can be addedas required to coincide with the back end of each portion of the partthat is to be created by a different charge. If such additional set ofports are included, they would be opened and closed in the same manneras the second set described above (optional step 506 of FIG. 8).

It is also noted that the "time exit" approach could be employed withthe injection molding of homogeneous parts that are relatively long,i.e. where there is a considerable distance between the end of the spruechannel(s) and the furthermost reaches of the mold cavity. Byincorporating at least a second set of resin outlet ports, for exampleat the midpoint of the mold cavity in relation to the sprue channel(s),the molding process can be expedited. The process would be essentiallythe same as that described for the injection molding of a heterogeneousstructure, except that only one charge of bulk molding compound isinvolved, and the second set of ports would be opened once the fiber andfiller had packed back to just forward of the second set of ports. Ifnecessary, additional sets of outlet ports can be added to furtherexpedite the injection molding of extremely long parts.

While the invention has been described in detail by reference to thepreferred embodiment described above, it is understood that variationsand modifications thereof may be made without departing from the truespirit and scope of the invention. For example, although the compressionand injection molds described previously contained only one part-formingcavity, this need not be the case. A compression or injection mold asemployed with the present invention could include multiple part-formingcavities.

Wherefore, what is claimed is:
 1. A method of making a fiber reinforcedceramic matrix composite (FRCMC) part comprising the steps of:(a)placing a quantity of bulk molding compound into a female die of a mold,the mold having at least one outlet port, said bulk molding compoundcomprising:a quantity of fibers which corresponds to the maximum percentby volume of fibers capable of being dispersed throughout the FRCMCpart; and an amount of pre-ceramic resin exceeding the percent by volumeof ceramic material desired to be in the FRCMC part, wherein excessresin exceeding that required to form the part is expelled through theat least one resin outlet port; (b) pressing a male die of the mold ontothe female die so as to displace the bulk molding compound throughout acavity formed between the female and male dies, said cavity having wallswhich form the exterior surfaces of the FRCMC part being molded; (c)heating the mold at a temperature and for a time associated with thepre-ceramic resin which polymerizes the resin to form a fiber-reinforcedpolymer composite structure; (d) removing the polymerized compositestructure from the mold; and (e) heating the polymerized compositestructure at a temperature and for a time associated with thepolymerized resin which pyrolyzes it to form a FRCMC structure.
 2. Themethod of claim 1, wherein the amount of pre-ceramic resin is sufficientto ensure the fibers flow with the resin within the cavity of the moldat its prescribed viscosity.
 3. The method of claim 2, wherein theprescribed viscosity of the pre-ceramic resin is chosen such that it ishigh enough to ensure the fibers flow therewith inside the cavity of themold, but low enough to ensure the resin readily flows through thefibers once packed into position.
 4. The method of claim 3, wherein thechosen viscosity of the pre-ceramic resin is greater for longer fibers.5. The method of claim 4, wherein said fibers are approximately the samelength, said fiber length being with a range of about 0.125 inch to 12inches, and wherein the prescribed viscosity of the pre-ceramic resin isbetween 5000 and 30,000 centipoise.
 6. The method of claim 1, whereineach resin outlet port has a cross-sectional area small enough tosubstantially ensure the fibers cannot flow through the port, yet largeenough to allow the flow of resin therethrough.
 7. The method of claim1, wherein the bulk molding compound further comprises a quantity offiller material which corresponds to the percent by volume of fillermaterial desired to be dispersed throughout the FRCMC part.
 8. Themethod of claim 7, wherein the maximum percent by volume of fiberscapable of being dispersed throughout the FRCMC part is between about 15and 50 percent dependent on the length of the fibers employed, the shapeof the part being molded, and the quantity of filler material.
 9. Themethod of claim 1, further comprising the steps of:(f) after thecompletion of step (e), immersing the FRCMC part containing pores formedduring step (e), into a bath of a pre-ceramic resin to fill the pores;(g) heating the FRCMC part at a temperature and for a time associatedwith the resin filling said pores so as to transform it to a ceramicmaterial; (h) repeating steps (f) and (h) until the pore density withinthe FRCMC part is less than a prescribed percentage by volume.
 10. Themethod of claim 9, wherein the resin has a water-like viscosity.
 11. Themethod of claim 9, wherein step (f) further comprises the step ofplacing the FRCMC part in a vacuum environment to facilitate the fillingof the pores.
 12. The method of claim 9, wherein the prescribedpercentage by volume of the pore density of the FRCMC part is such thatthe part exhibits a repeatable strength.
 13. The method of claim 1,wherein step (a) further comprises the step of placing a sheet of fibercloth on top of said quantity of bulk molding compound.
 14. The methodof claim 13, wherein the step of placing the sheet of fiber cloth ispreceded by the step of saturating the sheet with said preceramic resin.15. The method of claim 13, wherein step (a) further comprises the stepof placing a second quantity of bulk molding compound into a female dieof a mold on top of the sheet of fiber cloth.
 16. The method of claim 1,wherein step (a) further comprises the steps of:(a2) placing a sheet offiber cloth on top of said quantity of bulk molding compound; (a3)placing an additional quantity of bulk molding compound into a femaledie of a mold on top of the sheet of fiber cloth; and (a4) repeatingsteps (a2) and (a3) as desired.
 17. The method of claim 16, wherein thestep of placing the sheet of fiber cloth is preceded by the step ofsaturating the sheet with said preceramic resin.